*
 margin 0
 padding 0

.header
    height 100px
    background-color green
    h1
      font-weight: 700
      font-size: 26px
      color: blue
 
.footer
     background pink
     height 100px

main 
    display: flex
  nav,div,aside
    height 500px
    width 200px
    background-color yellow
   nav 
     background-color grey
   aside
     background-color red
   div 
     //flex: 1  //主要元素，其它元素占满后，剩下空间全部占满
     width calc(100% - 400px)